Dot onions with cheese. Slice remaining potatoes, and arrange a second layer in overlapping circles over onion mixture. Season lightly with salt and pepper, and brush with remaining tablespoon oil. Place skillet in the oven, and bake for 20 to 25 minutes, or until a toothpick goes through easily.

Let torta cool 2 to 3 minutes in the pan. Carefully run a small, sharp knife around the edge. Hold skillet with a clean kitchen towel, and invert torta onto a plate. Cut into wedges, and serve.